Baught faulty second hand car, 9 months in w/shop

I purchased a second hand Audi A4 in January of 2014. I drove the car for a week and then noticed my Check Engine light came on. I returned the car only to find out by the technician himself that he did not repair the fault, he just reset the computer. He gave me the car back after a few days saying the fault was fixed. On week passed and the same thing happened again, so I returned the car again. They \fixed"it and gave it back. I took the car back a THIRD time and they still did not fix it. The fourth time they apparently fixed it the car seemed fine"